This invention relates to a metallic sheath used for prestressed concrete structures constructed through a posttensioning method, and more specifically, to a metallic sheath provided with rust proofing treatment.
Generally, when a prestressed concrete structure is executed by a posttensioning method, metallic sheathes are normally used because a tendon is arranged. The metallic sheath has a function as a cover for the tendon so that the tendon is insulated from concrete and thus can be tensioned.
The metallic sheath allows the tendon to be arranged smoothly, and have strength enough to withstand a collapse thereof or formation of holes therein when concrete is placed.
Frictional resistance must be small when the arranged tendon is prestressed. The inventors of this invention has invented a metallic sheath whose inner peripheral surface is coated with a coating in order to solve the above problem and has filed a patent application, U.S. Ser. No. 441,080, now allowed with USPTO.
In addition to the above problem on a metallic sheath, however, rust-proofing effect (corrosion resistance) thereof has become an important problem lately.
Cases are getting conspicuous recently that when the concrete structures like the prestressed concrete structures (hereinafter referred to as PC structure) and the reinforced concrete structures (hereinafter referred to as RC structure) are built under sea water or adjacent to the seashore, or when shore sand is used for concrete aggregate material, the tendon within the concrete is corroded at a speed quicker than expectation and therefore the concrete structures themselves are damaged through the corrosion caused by the abound existence of negative ion like halogen ion (for example Cl.sup.-, Br.sup.-, I.sup.-, SO.sub.4.sup.--, S.sup.-- and others), especially, of chlorine ion.
In the PC structure cases, the metallic sheath covering the tendon is directly corroded by negative ion (called as corrosion ion) so that, centering said corroded sheath, the concrete as well as the tendon around said sheath are also damaged or corroded, which causes the infirmity of whole PC structures.
In these PC structures, in addition, electrochemical corrosion occurs by the contact or the close access between the metallic sheath and the tendon and fosters the infirmity of the PC structure.
In the prior art, rust-proofing treatment such as zinc plating was applied upon the metallic sheath to prevent them from rusting when they were to lay aside for several months at the seashore or places like that or when it required a long time before they were imparted with prestressing operation after they were arranged.
In this prior art, however, the main purpose was to reduce the friction between the metallic sheath and the tendon therewithin, or merely to prevent from rusting which would increase the frictional resistance. It was not considered at all about the corrosion caused by the permeation of chlorine ion and the like during a long time of period after the concrete was placed.
Said zinc plating has no effect to control the electrochemical corrosion generated by the contact or the close access between two different kinds of metals in chlorine ion atmosphere.
Moreover, when the inner peripheral surface of the sheath began to corrode by said electrochemical corrosion or the corrosion caused by the peeling-off of rust-proofing-layer by tendon during prestressing operation, the corrosion advanced toward the outer peripheral surface of the sheath, then progressed at an accelerated speed. There was no effective countermeasure to prevent this corrosion starting from the inner peripheral surface of the sheath so far.